This invention relates to electronic devices and more particularly, to portable electronic devices with satellite navigation system capabilities.
Electronic devices use satellite navigation systems to support navigation functions. For example, an electronic device may use a satellite navigation system such as the Global Positioning System (GPS) to obtain position information, timing information, and other navigation information. The Global Positioning System includes satellites that orbit the Earth, Earth-based control and monitoring stations, and GPS receivers that are located within the electronic devices. GPS services may be provided on a continuous basis anywhere that is within range of the orbiting satellites.
A portable electronic device may include a GPS receiver. The GPS receiver may sometimes be referred to as a GPS unit. The GPS unit determines the current position (location) of the portable electronic device. During operation, the GPS unit may receive data streams from GPS satellites orbiting the Earth. Using a local clock, the GPS unit analyzes each data stream to make a transit time and distance estimation.
A method known as geometric trilateration may be used to determine the location of the GPS unit by analyzing the estimated distances of each of the satellites to the GPS unit. The accuracy of location measurements made using the GPS unit depends on accuracy of the local clock. The local clock is typically implemented using a crystal oscillator. If the output of the oscillator exhibits errors, the GPS receiver may not function as expected.
Some GPS units are housed in dedicated handheld devices. Other GPS units are used in more complex devices such as cellular telephones. Devices such as these may have components whose operations can adversely affect GPS performance.
As an example, a cellular telephone may include cellular telephone transceiver circuitry that is used to make telephone calls. The cellular telephone transceiver circuitry includes power amplifier circuitry that transmits radio-frequency (RF) signals to a nearby base station. If care is not taken, a rapid change in heat generated from the power amplifier circuitry may adversely affect the accuracy of the oscillator in the GPS unit, thereby resulting in degraded GPS performance. Acquiring a GPS location measurement when making a phone call may therefore be unacceptably slow.
Conventional arrangements for testing GPS receiver performance involve measuring the performance of the GPS unit while the power amplifier circuitry is placed in an active mode that constantly transmits radio-frequency signals. The performance of the GPS unit, however, may be most adversely affected when the thermal transient (i.e., the instantaneous change in heat generated by the cellular telephone transceiver circuitry) is maximized. Testing GPS performance using the conventional approach is not a rigorous test of GPS performance, because leaving the power amplifier circuitry in the active mode does not maximize thermal transient.
It would therefore be desirable to be able to provide ways of testing GPS receiver performance.